An optical transmitter module which converts an electric signal to an optical signal is used in optical communication systems. Particularly, an optical transmitter module used for long-distance transmission, for example for several tens of kilometers or more, includes a light source, an optical modulator for modulating CW light output from the light source, and a driving circuit for driving the optical modulator. Generally, a laser diode or the like is used as the light source, and an EAM (Electro Absorption Modulator), an MZM (Mach-Zehnder Modulator), or the like is used as the optical modulator.